Open topped cargo compartments, such as dump trucks, short vehicle trailers, storage bins and the like, are subject to having material stored therein fall or blow out. This can be particularly dangerous while the compartment is being transported. For instance, if portions of the load fall out onto a roadway (e.g., trucks or trailers hauling sand, gravel, dirt or similar materials), traffic accidents can occur, as well as damage to other vehicles and littering of the roadway. Wind effects can intensify the risk of inadvertent material loss. For instance, lightweight materials, such as plant clippings being taken to a landfill, mulch materials, or harvested plants like cotton, are liable to having gusty winds blow portions of the contents out of the vehicle or bin.
In many places, laws and regulations require the tops of such containers to be covered before they can be transported on public roadways to prevent material being blown out of the containers. Even where there are no laws regarding covering a load, it is beneficial for the driver to cover the load to reduce the possibility of damage to other property from escaping materials. Thus, tarpaulins are commonly used to cover the loads during transport. In addition, tarpaulins are often used to keep environmentally sensitive materials, such as asphalt, wheat, etc. dry and warm or cool during static storage as well as during transport.
Manually placing a tarp or cover on an open-top container is both time-consuming and expensive. As such, automated tarpaulin covering systems are commonly used. FIG. 1 shows a conventional automated tarpaulin covering system 10 for a cargo container 12. The covering system 10 has a tarpaulin 14 connected at one end to a movable cross bar 16. The cross bar joins parallel arms 18 and 20 that are pivotally mounted on opposite sides of the cargo compartment via pivot 22. The parallel arms are spring biased rearward toward a covered position. The tarpaulin 14 is rolled on a roller bar 24, from which it extends to connect with cross bar 16. Roller bar 24 is mounted at the top of the cab shield 26 of the container. Roller bar 24 is driven via electric, hydraulic or manual mechanisms to roll and unroll the tarpaulin 14. Due to their rearward spring bias, the parallel arms pull the tarpaulin toward the rear of the truck as it is unrolled, which covers the opening on top of the container.
The spring-driven force applied to the tarpaulin via parallel arms 18 and 20 decreases as the parallel arms rotate rearward to the covered position, because of the nature of the spring biasing that drives the parallel arms. The reduced spring force applied at the end of the tarp-covering stroke results in the parallel arms having reduced force to maintain tension in the tarpaulin in the covered position. Thus, the tension applied to the tarpaulin in conventional systems is often ineffective for counteracting movement of the stored materials against the inside of the tarpaulin, wind gusts applied to the tarpaulin, or other forces applied during static storage conditions.
During transport of the container, the low tension applied to the tarpaulin is further exploited by higher wind shear and aeronautic effects. When traveling, trucks hauling these containers create turbulent airflow at their headend that undulates the tarpaulin as the turbulent air passes over it. In addition, low pressure on the open side of the container creates lift on the tarpaulin, which acts like an airfoil. The turbulent air and low pressure periodically lift the tarpaulin upwards when the vehicle is in motion. The reduced spring tension coupled with the undulating, upward motion of the tarpaulin often permits the tarpaulin to expose partially the open top container and, in some extreme cases, can catastrophically damage the arms, tarpaulin and other components of the automated tarpaulin system.
Manual and automatic hold down mechanisms are known that attempt to improve tension in the tarpaulin while in the closed configuration. The conventional automated systems rely upon complicated arrays of mechanical, electrical or hydraulic structures to apply a constant tension along the length of the tarp. In addition, the conventional systems typically must be activated by the user as a separate step subsequent to covering the container. Further, the conventional systems often require activation energy, such as electrical and hydraulic inputs, beyond movement used to move the cover.
The most common conventional system is a manual tie down system of cords attached to either the tarpaulin or the parallel arm structure of the automated tarpaulin system to improve tension in the tarpaulin and its retention of material within the container. The manual tie down system requires the vehicle operator to climb to the open top container and secure the cords after the cargo is loaded and the automated tarpaulin system has deployed. It also requires undoing the cords just before the tarpaulin is uncovered prior to unloading the cargo. This is very time consuming and potentially a hazardous process for the vehicle operator.
Another conventional hold down mechanism is shown in U.S. Pat. No. 6,234,562 issued May 22, 2001 to Henning (Henning), which discloses a lock mechanism that relies upon wind energy to activate it. The lock mechanism includes a pivotally mounted wind vane that rotates in response to sufficient air pressure being applied against the vane to activate the mechanism, such as would be provided at a certain rate of travel on a roadway. In addition, Henning teaches that the lock mechanism can be deployed by a human via activation of a cable system or of a mechanical linkage system using an electrically or hydraulically driven actuator. Thus, the Henning system requires sufficient air pressure to deploy the lock mechanism or it requires the user to activate another system to deploy it.